1. Field of the Invention
The present invention relates generally to a magnetic bearing protection device and a turbomolecular pump and, more particularly, to a magnetic bearing protection device and a turbomolecular pump in which stable control can be achieved in light of excessive oscillation of a rotator and external vibrations.
2. Description of the Related Art
FIG. 3 is a structural diagram shoving a conventional turbomolecular pump with a 5-axes-control type magnetic bearing. Magnetic bearings are for floating a rotor (hereinafter referred to as a rotator) by a magnetic force to hold the rotator without touching it. Being capable of rotating a rotator at a high speed and suitable for use in an environment required to be highly clean, magnetic bearings are used in technical fields relating to the manufacture of semiconductors, for example, as bearings for holding rotor blades of turbomolecular pumps.
In a 5-axes-control type magnetic bearing, radial position controlling electromagnets 3A, 3B and an axial position controlling electromagnet 5 are used to float rotor blades 1.
For instance, the radial position of the upper rotor blades 1 is actively controlled such that a present radial position of the upper rotor blades 1 is detected by a radial position sensor 7A and, through a compensating circuit 9A having a PID adjusting function, after current is amplified by a current amplifier 11A, the radial position controlling electromagnet 3A is driven.
The radial position of the lower rotor blades 1 and the axial position of the rotor blades 1 are controlled by a similar mechanism, though not shown.
With the rotor blades 1 being accelerated by a high frequency motor 21, a temporal oscillation may take place when passing through a so-called resonance point unique to the rotor blades 1, where their unique frequency coincides with the rotation frequency. In addition, the rotor blades 1 may be shaken from an externally given shake such as an earthquake, or from a sudden gas (the atmosphere) suction for some reasons while the turbomolecular pump discharges gas from a not-shown chamber through an inlet port 19, which results in shift from vacuum to atmosphere release (hereinafter referred to as atmosphere burst-in).
How the rotor blades 1 oscillate upon this atmosphere burst-in is illustrated in FIG. 4. Shown in the figure is a record of the behavior in the axial direction of the rotor blades 1, expressed by outputs of the axial position sensor incorporated in the turbomolecular pump. The rotor blades 1 are, for an instant, depressed to the outlet port side upon the atmosphere burst-in, but are immediately pushed upward to the inlet port 19 side by an aerodynamic thrust as in a helicopter.
As shown in FIG. 4, the rotor blades 1 gradually stop oscillating in about a few seconds (6 seconds, in this case) after the atmosphere burst-in. When such oscillation takes place, the rotor blades 1 come into contact with touch down bearings 13, 15, and might bring them along and wear or damage the touch down bearings 13, 15 in some cases.
The attempt to bring the rotor blades 1 in such state back to a normal rotation position by floating it with a magnetic force causes excessive current, for the current is increased in radial position controlling electromagnets 3A, 3B and the axial position controlling electromagnet 5. Because of this excessive current, a coil for magnetic bearing is overheated. A conventional countermeasure against such overheat is to provide a magnetic bearing driver circuit with a current limiter 17 to limit the current flowing into the coil for magnetic bearing to, for example, 2 [A].
As described above, current is always limited in a conventional turbomolecular pump irrespective of operation condition of the pump. This limitation leads to a power shortage of the magnetic bearing against the above-mentioned excessive oscillation of the rotor blades 1 and externally given shake, delaying the end of oscillation of the rotor blades 1. This delay of the end increases the load put on the touch down bearings 13, 15.
The present invention has been made in view of the above problems inherent in the prior art, and an object of the present invention is therefore to provide a magnetic bearing protection device and a turbomolecular pump in which stable control can be held against the excessive oscillation of a rotator and an externally given shake.
In order to attain the object above, a magnetic bearing protection device of the present invention comprises:
a position sensor for detecting the position of a rotator;
at least a pair of controlling electromagnets capable of holding the rotator in the air and of controlling the radial position or the axial position of the rotator based on a position signal obtained from the detection made by the position sensor;
a current supplying means for supplying electric current to the controlling electromagnets;
a controlling current detecting means for detecting the current flowing into the controlling electromagnets from the current supplying means;
a current anomaly judging means for judging whether or not the current detected by the controlling current detecting means exceeds a set current value that is set in advance;
a time anomaly judging means for judging, when the current anomaly judging means judges that the flowing current exceeds the set current value, whether or not the current exceed lasts for over a set time that is set in advance; and
a stopping means for stopping current supply from the current supplying means to the controlling electromagnets when the time anomaly judging means judges that the lapse of time from the start of the current exceed is longer than the set time.
The position sensor detects the position of the rotator. This position sensor can detect the radial position or the axial position of the rotator. The controlling electromagnets can hold the rotator in the air and control the radial position or the axial position of the rotator based on a position signal obtained from the detection made by the position sensor.
The controlling electromagnets, in the case of a 3-axes-control type magnetic bearing, for example, consist of a pair of radial position controlling electromagnets and a pair of axial position controlling electromagnets. In the case of a 5-axes-control type magnetic bearing, the controlling electromagnets consist of two pairs of radial position controlling electromagnets which are axially spaced apart from each other and a pair of axial position controlling electromagnets.
Alternatively, some of the radial position controlling electromagnets and of the axial position controlling electromagnets may be permanent magnets. A bearingless motor or the like may be used which produces, while adjusting magnetically the radial position, a rotational force through a magnetic force that is generated from an electromotor coil and is unbalanced by a magnetic force generated from the radial position controlling electromagnets.
The current supplying means supplies the controlling electromagnets with current. The controlling current detecting means detects the current flowing from the current supplying means to the controlling electromagnets. The current anomaly judging means judges whether or not the current detected by the controlling current detecting means exceeds a set current value that is set in advance.
The time anomaly judging means judges, when the current anomaly judging means judges that the flowing current exceeds the set current value, whether or not the current exceed lasts for over a set time that is set in advance. The stopping means stops current supply from the current supplying means to the controlling electromagnets when the time anomaly judging means judges that the lapse of time from the start of the current exceed is longer than the set time.
In short, when the current flowing into the controlling electromagnets reaches a preset value for a given period of time, the magnetic bearing is stopped for the purpose of protecting the same. However, no limitation is put on the value of the current flowing into the controlling electromagnets, so that oscillation of the rotator, etc., will end early in various conceivable events.
In most cases, therefore, the situation will be recovered before the magnetic bearing has to be stopped.
Also, the early ending of the oscillation of the rotator gives less damage to the touch down bearings.
According to another measure to attain the above object, a turbomolecular pump of the present invention comprises:
rotor blades;
a rotating means for rotating the rotor blades;
a position sensor for detecting the position of the rotor blades;
at least a pair of controlling electromagnets capable of holding the rotor blades in the air and of controlling the radial position or the axial position of the rotor blades based on a position signal obtained from the detection made by the position sensor;
a current supplying means for supplying electric current to the controlling electromagnets;
a controlling current detecting means for detecting the current flowing into the controlling electromagnets from the current supplying means;
a current anomaly judging means for judging whether or not the current detected by the controlling current detecting means exceeds a set current value that is set in advance;
a time anomaly judging means for judging, when the current anomaly judging means judges that the flowing current exceeds the set current value, whether or not the current exceed lasts for over a set time that is set in advance; and
a stopping means for stopping current supply from the current supplying means to the controlling electromagnets when the time anomaly judging means judges that the lapse of time from the start of the current exceed is longer than the set time.
The pump of the present invention is an application of the above magnetic bearing protective device to a turbomolecular pump.
With the structures above, stable control can be held against the excessive oscillation of the rotor blades and externally given shake.